1. Field of the Invention
The present invention relates to a composition for peelable coating, and more particularly to an aqueous composition for peelable coating that suitably protects the appearance of an object temporarily and forms a coating film having excellent physical properties and peelability.
2. Background Art
Before delivery to customers, automobiles and other vehicles (hereinafter simply referred to as vehicles) are usually stored in an open-air stock yard, or transported under their own power or by means of a cargo train, a trailer, or a ship. During storage or transportation, vehicles may be damaged due to contact with other objects or blemished (e.g., spotting, discoloring, staining, aging) by agents such as sand dust, iron powder, salts, alkalis, and acids; smoke; bird droppings; dead insects or the body fluids of insects; sunlight; or by the elements, such as acid rain.
Once a vehicle is damaged or stained, the commercial value thereof decreases considerably. In order to prevent this, vehicles must be temporarily protected for a certain period of time. Under a known conventional method, upon shipping, protective film prepared from a composition that forms peelable coating is applied to such objects, and the film is removed after the storage period is over.
Other than vehicles, a variety of objects, including ferrous or nonferrous metallic articles such as mechanical parts; houseware; wood articles; glass products; plastic products; and rubber products (hereinafter simply referred to as objects) are stored indoors or outdoors over the distribution period. In order to maintain the commercial value thereof, formation of a protective film on a surface of an object has been proposed.
Such a protective film must meet the following requirements 1) to 5).
1) The protective film must be able to adhere to an object in such a way that the film does not spontaneously peel off the object during storage yet can be easily peeled off when necessary.
2) The protective film covering an object must have excellent physical properties; e.g., tensile strength and elongation, such that the film be can easily peeled off the object without breaking the film.
3) The protective film covering an object must have excellent film properties; e.g., water resistance, light resistance, and thermal stability, such that film characteristics and peelability do not vary during storage of the object.
4) The protective film covering an object must protect the object both chemically and physically.
5) The protective film covering an object should not affect the object detrimentally.
The present inventors have already proposed an aqueous dispersion of a peelable coating composition which provides protective film and satisfies the aforementioned characteristics (See U.S. Pat. No. 6,211,282). The said patent discloses an aqueous dispersion of a peelable coating composition providing protective film, which dispersion is formed of a mixture of an emulsion containing an acrylic copolymer having a glass transition temperature of 40xc2x0 C. or higher and an emulsion containing an acrylic copolymer having a glass transition temperature of 5xc2x0 C. to xe2x88x9220xc2x0 C. The coating satisfies the aforementioned requirements.
However, the composition for peelable resin coating when applied to a surface of an object must be heated; e.g., at approximately 70-80xc2x0 C., so as to form a protective film and develop the aforementioned characteristics required of the protective film.
As in similar cases of conventional compositions for peelable coating, the resin composition exhibits physical properties such as mechanical strength and elongation of film brought about through heating at a high temperature. Drying of the resin composition at ambient temperature requires a long period of time, and the performance of the dried film is poorer than that of a high-temperature-heated product.
In addition, when the composition is dried at ambient temperature, the formed protective film has poor water resistance and absorbs water if rainfall is prolonged. Thus, the protective film detaches from the object and peels off the object under its own weight.
When the aforementioned resin composition for peelable coating is applied at a low temperature, film-formability is poor and in some cases the composition per se cannot be used.
In the present invention, the term xe2x80x9cambient temperaturexe2x80x9d is not particularly defined. However, the term refers to temperatures higher than 0xc2x0 C.; i.e., temperatures at which water does not freeze during operation, generally 5-35xc2x0 C.xc2x15xc2x0 C.
In recent years, there has been increasing demand for protective coating film that can be applied to a wide range of objects; i.e., from small objects to automobiles. Thus, the protective film must be usable under a wide range of conditions for application and storage. Such circumstances require that a composition for peelable coating have excellent properties such as film-formability and water resistance and can be dried at ambient temperature without the need for particular heating, thereby providing coating performance.
In order to solve the aforementioned drawbacks, an object of the present invention is to provide a composition for peelable coating, which composition has excellent properties such as film-formability and water resistance and can be dried at ambient temperature without the need for particular heating, thereby providing satisfactory coating performance.
In view of the foregoing, the present inventors have conducted extensive studies in order to solve the aforementioned drawbacks, and have found that an excellent composition for peelable coating can be obtained from a copolymer comprising at least two acrylic copolymer portions having a specified glass transition temperature range and prepared through multi-step polymerization. The present invention has been accomplished on the basis of this finding.
Accordingly, the present invention provides an aqueous composition for peelable coating, which forms peelable coating for protecting a surface of an object for a specific period of time and can be peeled off after providing necessary protection, which composition contains a core/shell copolymer comprising, in the molecule thereof, an acrylic copolymer portion A having a high glass transition temperature of 30xc2x0 C.-70xc2x0 C. and an acrylic copolymer portion B having a low glass transition temperature of 5xc2x0 C. to xe2x88x9230xc2x0 C. and being formed through multi-step polymerization.
Preferably, particles of the core/shell copolymer has a continuous dual-phase structure.
Preferably, the core is made up of the copolymer portion A and the shell is made up of the copolymer portion B.
Preferably, the core/shell copolymer comprises 5-40 wt. % of the acrylic copolymer portion A and 60-95 wt. % of the acrylic copolymer portion B, on the basis of the total amount of the portion A and the portion B.
Preferably, each copolymer portion contains a hard-polymer-providing monomer unit and a soft-polymer-providing monomer unit in a total amount of 70 wt. % or more.
Preferably, neither the acrylic copolymer portion A nor the acrylic copolymer portion B contains a monomer unit formed of a nitrogen-containing vinyl monomer.
Preferably, a vinyl monomer having an acidic functional group is contained in an amount of 0.2-2 wt. % based on the total amount of monomers.
Preferably, a reactive surfactant is used during formation of an emulsion of the acrylic copolymer portion A or the acrylic copolymer portion B.
Preferably, the composition contains a peelability-enhancing agent.
The present invention also provides a peelable coating film which can be easily peeled off an object and is obtained from a composition for peelable coating as recited above.
The present invention also provides an object having a peelable coating film which can be easily peeled off the object and is obtained from a composition for peelable coating as recited above.
The present invention also provides a method for protecting the surface of an object, which method comprises forming, on the surface of the object, a peelable coating film obtained from a composition for peelable coating as recited above.
Preferably, in the method, the peelable protective coating film is formed at 40xc2x0 C. or lower.